The present invention relates to a method and system of controlling an electromechanical axial setting device, particularly suited for friction couplings.
The invention relates to a method of returning an electromechanical axial setting device, wherein the axial setting device includes the following: two setting rings centered on a common axis, one of which is axially held, with the other one being axially displaceably mounted, and one of which is rotationally fixedly held in a housing, with the other one being rotatingly drivable. The two setting rings, on their respective end faces facing one another, each include an identical plurality of circumferentially extending grooves. The grooves, in a plan view of the end faces, have depths which rise in the same circumferential direction. Pairs of grooves in the two setting rings each accommodate a ball. The rotatingly drivable setting ring is connected to an electric motor in respect of drive, and the axially displaceable setting ring is loaded by pressure springs towards the axially held setting ring. When applying a positive voltage to the electric motor, the axial setting device moves into an advanced position, and when disconnecting the voltage from the electric motor, the axial setting device returns into a starting position.
Furthermore, the invention relates to an electromechanical axial setting device for carrying out the inventive method.
Axial setting devices of the foregoing type have a simple and compact design and comprise short reaction times, such as they are required, for example, in friction couplings in locking differentials. The use of such setting devices is described in DE 39 20 861 C2, DE 39 15 959 C2, DE 39 09 112 C2, DE 38 15 225 C2 and DE 100 33 482.2. In these publications, it is mentioned several times that to make locking differentials comprising such setting devices compatible with vehicles provided with ABS systems and/or ESP systems, it must be possible for such axial setting devices to be returned quickly. Such a return motion is achieved by return springs. When in the form of spiral springs, they rotate the rotated setting ring backwards directly and thus allow the axially displaced setting ring to return. When in the form of axial springs, with groove assemblies without self-inhibition, they push back the axially displaced setting ring and thus rotate the rotated setting ring backwards.
Axial setting devices of the foregoing type, especially those wherein reinforced return springs or voltage reversing circuits effect a rapid return for the purpose of achieving a rapid disconnection of the friction coupling, at the completion of the return motion, experience a hard jerk due to the balls hitting the groove ends of the ball grooves of the setting ring. This jerk is so pronounced that in a vehicle it is regarded as an unacceptable adverse effect on the comfort conditions in the vehicle. Furthermore, if the driver is unprepared for such a jerk, it can make the driver feel insecure and cause him to regard the jerk as damage to the vehicle.
It is therefore necessary to achieve a rapid return motion and a dampened stopping behavior. It would therefore be desirable to provide a new method of returning an electromechanical axial setting device, especially for friction couplings.
The present invention provides an improved method and system for controlling an electromechanical axial setting device. The present invention provides that, during the return motion, shortly before the starting position is reached, the electric motor is short-circuited for the purpose of generating a braking moment. A further method in accordance with the invention provides that, during the return motion, shortly before the starting position is reached, a positive voltage is briefly applied to the electric motor for the purpose of generating a braking moment. At the end of the returning process, shortly before the balls reach the end stops, the means described here thus offer an electric braking method which can be achieved with very few additional switching mechanisms, while the basic mechanical configuration can remain unchanged.
A device in accordance with the invention includes a rotational position sensor which is arranged at one of the rotating parts, e.g. at the first setting ring and which, shortly before the balls reach the end stops in the ball grooves, initiates the respective switching operation, i.e. it either initiates short-circuiting or applies a positive voltage. In one embodiment it is also possible to replace the non-contact sensor by switching contacts which first, shortly before the end stops are reached, effect short-circuiting or the application of voltage and which, when the balls reach the end stops in the ball grooves, disconnect the electric motor. The means for dampening the stopping effect and for braking the returning motion of the electric motor constitute much simpler means than would be required by mechanical brakes and/or damping devices.
To accelerate the return motion, a negative voltage may be applied to the electric motor for returning purposes. For accelerated control cycles, this method can be improved further by the following steps. For returning purposes, first a negative voltage is applied to the electric motor and when the idling speed of the electric motor has been reached, the electric motor is disconnected from the voltage.
By way of the control strategy described herein it is possible to save a great deal of time as compared to the simple passive return motion initiated by spring force in that, especially during the acceleration phase, the spring force is supported by the drive effected by the electric motor. Surprisingly, time is also saved as compared to a permanently applied negative voltage during the return motion, the effect of which permanently applied negative voltage is counteracted by the induced counter voltage if the idling speed is exceeded.
Another embodiment includes a voltage reversing circuit for the electric motor and a motor speed recording device for the electric motor which are logically connected to one another via the idling speed of the electric motor in such a way that the voltage reversing circuit is disconnected if, in the course of the device being returned, the idling speed of the electric motor is reached.
To be able to determine the point in time at which the negative voltage at the electric motor is interrupted, it is possible to use direct speed monitoring means. However, if the dynamic behavior of the axial setting device is known, a simple time switch can be used for limiting, in terms of time, the connection time of the negative voltage at the electric motor.
In a preferred embodiment, use is made of a permanently excited direct current motor.
Other advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.